As generally known in the art, X-rays were first observed and documented in 1895 by Wilhelm Conrad Roentgen, a German scientist. At that time, X-rays were used as a radiograph for detecting an internal structure of an object. In 1912, a German physicist named Laue discovered diffraction of X-rays. That is, X-ray diffractometry was established based on the fact that crystal reflects X-ray irradiated into the crystal if the X-ray has a wavelength corresponding to a plane spacing of the crystal. The X-ray diffractometry demonstrates the wave nature of the X-ray and the regular alignment of atoms in the crystal.
In the same year (1912), a British scientist named Bragg suggested conditions required for the X-ray diffraction by analyzing the X-ray diffractomerty and developed Bragg's law to simply explain X-ray scattering from atomic planes of the crystal.
The X-ray diffractometry is useful for understanding the alignment and relative positions of atoms provided in a substance having a relatively simple structure and physical properties of metals, polymeric materials and solid materials. Recently, the X-ray diffractometry is extensively used for analyzing internal structures of complex natural substances, such as steroid, vitamin, or antibiotic substances.
The X-ray is generated when a charged particle having sufficient energy suddenly stops its movement. In general, the X-ray is created in an X-ray tube having an electron source and two metal electrodes. Since high voltage (tens of thousands of volts) is applied between two metal electrodes, electrons are emitted from a cathode of the electron source due to the high voltage. The electrons may collide with a metal target provided at an anode of the electron source with a high speed, thereby generating the X-ray. Such an X-ray source may create a continuous spectrum and a linear spectrum. Accordingly, the X-rays are classified into continuous X-rays or white X-rays representing the continuous spectrum and characteristic X-rays representing the linear spectrum. The continuous X-rays and characteristic X-rays are very important when analyzing properties of substances.
The continuous X-ray is called “white radiation ray” or “Bremsstrahlung”, which is generated when a part of energy of the electrons colliding with the metal target is converted into X-ray photons.
Referring to an X-ray emission spectrum, a smooth curve of the X-ray emission spectrum is suddenly peaked at a predetermined wavelength. An X-ray having a wavelength representing the peak is called a “characteristic X-ray”. The wavelength of the characteristic X-ray may vary depending on substances and several characteristic X-rays are observed in the same substance. The wavelength of the characteristic X-ray has a characteristic value, which may vary depending on elements forming a target material, and is classified into various classes, such as K, L, M, etc, according to the energy level of the electrons. At this time, the size of the wavelength is in an order of K<L<M . . . , and several wavelengths may exist in the same class.
An atom consists of a nucleus and an electron moving around the nucleus along an orbit with a predetermined energy level. If kinetic energy of the electron used for generating the X-ray is larger than bonding energy of the electron moving along the orbit with the predetermined energy level, the electron in the predetermined energy level is emitted. In addition, an electron aligned in an outer energy level is excited and moved into an empty space of the predetermined energy level. At this time, the characteristic X-ray is generated in each energy level.
The term “reflectivity” signifies an intensity ratio of reflected light to incident light and is utilized as an index showing the property of substances. Such reflectivity, which represents a reflection degree of light or radiation from a surface of an object, is determined according to the kind and the surface state of substances. In general, metals represent higher reflectivity. For instance, copper has reflectivity of 59% and silver has reflectivity of 95%. If an object is coated with black material, the object represents lower reflectivity due to a higher absorption characteristic of the black material. For example, soot absorbs 95% of light and reflects 5% of light. In addition, the reflectivity relates to the wavelength of incident light.
Crystal is used for reflecting the X-ray. The crystal plays the role of a mirror used for reflecting the visible ray. The reflectivity is one of important characteristics of the crystal. Thus, it is very important to rapidly and precisely measure the reflectivity of the crystal when performing X-ray tests or dealing with X-ray appliances.
Conventionally, the characteristic X-ray is used for measuring the reflectivity of the crystal.
That is, the characteristic X-ray generated from an anode of an X-ray tube is irradiated onto a sample crystal by means of a monochromator, and then reflection intensity of the characteristic X-ray is measured in order to obtain the reflectivity of the crystal. The reflection intensity of the characteristic X-ray reflected from the sample crystal can be obtained by measuring the number of photons using a scintillator, which is an X-ray detector, or a PM tube.
However, in order to allow the characteristic X-ray to be reflected from the surface of the sample crystal, constructive interference must occur between reflective waves in internal and external portions of the sample crystal. For this reason, in order to measure the reflectivity of the sample crystal by using the characteristic X-ray, it is necessary to find a specific incident angle of the characteristic X-ray while adjusting the incident angle of the characteristic X-ray with respect to the sample crystal in such a manner that the constructive interference occurs between reflective waves. In addition, in order to measure reflectivity of the X-rays with different orders of reflections, incident angles (Bragg angles) of the X-rays must be changed whenever they are incident into the crystal while rearranging the total system.
In addition, according to the conventional measurement method, since a cross sectional area of the characteristic X-ray beam irradiated onto the test sample is very small (about 1 mm2), it is necessary to rotate the sample crystal in order to find a spot incurring maximum reflectivity. Furthermore, the conventional measurement method uses the characteristic X-ray for measuring the reflectivity of the sample crystal, so an amount of available X-ray energy is limited. In addition, it is necessary to scan planes of large-sized crystal with the characteristic X-ray by several times in order to measure reflectivity of the large-sized crystal.